That's exactly the page I was about to post. Best to avoid a firmware modded to avoid battery drain. You'll need to find the procedure for starting a downgrade.
You might lose maps and music on the hard drive doing this, but got to be worth the risk at this point. If this doesn't work, then go back to 2760 but that will 100% wipe your maps.
It is definitely not a D unit so 4020 is about as high as it could go.
Good luck